Can you imagine that a cylindrical polymer rod with a diameter of 4.6 cm and programmed
height of 11 cm can jack up a car by simple heating? This can be done by our new shape
memory polymers (SMPs) which is designed by a new energy storage mechanism.

Thermoset shape memory polymers have been a topic of intensive research for years.
In addition to shape memory, which means a deformed temporary shape can restore its
original permanent shape upon stimulation, such as heat, light, moisture, pH, etc.,
SMPs can also release stress if free shape recovery is not allowed. The fact that
SMPs can memorize both shape and stress has rendered them with many potential applications
such as actuators, self-healing, sealants, morphing structures, stent, suture, soft
robot, smart textile, etc.

However, low output in stress and energy in the rubbery state has been a bottleneck
for wide-spread applications of thermoset SMPs. In the literatures, the stable recovery
stress is from tenths to several MPa and the energy output is several tenths MJ/m3. This suggests that the energy released by a 1cm3 sample can only lift 4 bottles of water (500g/bottle) by 1cm, a capability similar
to a baby.

They believe that if more energy can be stored during the deformation process, or
programming, more energy output can be achieved. Currently, entropy reduction is the
acknowledged mechanism for energy storage in thermoset polymers. In addition to entropy
reduction, they believe that enthalpy increase due to chemical bond length change
is an option. Conventional polymer networks cannot store energy through enthalpy increase
because the coiled segments in the rubbery state can endure giant deformation or ordering
or entropy reduction without stretching the chemical bonds, or if the bond is stretched,
it causes fracture. Hence, enriching the steric hindrance of the polymer network is
a solution. Following this idea, a commercially available epoxy (EPON 826) is reacted
with a rigid diamine named isophorone diamine (IPD), which can provide a large steric
hindrance. A stable recovery stress of 17 MPa and energy output of 2.12 MJ/m3 in rubbery state and in bulk form are obtained and largely maintained.